<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    
</body>
<script>
    //转字符型：数值，布尔，null,未定义
    let num1 = 123,flag1 = true,flag2 = false,num2 = null,num3;
    //string(),tostring()
    let str1 = String(num1);
    console.log(str1); 
    //console.log(String(num1)); //123
    console.log(typeof(str1)); 
    //console.log(num1.toString());  //123
    str1 = num1.toString();
    console.log(str1);
    console.log(typeof(str1));
    console.log('------------');
    console.log(String(flag1));
    console.log(flag1.toString());
    console.log(String(flag2));
    console.log(flag2.toString());
    console.log('------------');
    console.log(String(num2));
    //console.log(num2,toString());  //null没有toString()方法
    console.log(String(num3));
   // console.log(num3,toString());  //undefined没有toString()方法
</script>
</html>